1. Field of the Invention
The invention relates to a discharge lamp.
2. Description of the Prior Art
Discharge lamps are, for the most part, divided into lamps of the short arc type with a small distance between a cathode and an anode and into lamps of the long arc type with a large distance between the cathode and anode. A xenon lamp and a mercury high pressure lamp are known short arc lamps.
A short arc lamp, generally, is comprised of a roughly spherical emission part and tube arms located on both ends of the emission part. In the interior of the emission part is a pair of electrodes which are spaced a predetermined distance apart, and within the tube arm are a lead pin and a metal foil.
In the production of a lamp, it is necessary for purposes of encapsulation of a desired gas in the emission part, to install a gas tube which is used especially for this purpose. Since the tube arm, however, undergoes processing under heat, it is difficult to install the gas tube therein. The gas tube is, therefore, generally installed in one part of the emission part, and the gas is introduced and encapsulated in the emission part through this gas tube. After encapsulation, the gas tube is removed from the emission part. In this way, the desired gas can be encapsulated in the emission part and a hermetically sealed arrangement of the emission part can be obtained in which the encapsulated gas does not leak, and at the same time, in the tube arm, there can be a lead pin as a power supply lead and a metal foil.
However, it is difficult to completely remove the gas tube from the surface of the emission part. There always remains a residue of the gas tube on the surface of the emission part. This residue generally has a size of roughly a few millimeters; however, this residue produces refraction of light radiated from the emission part, and it can appear on an irradiated surface as a shadow. Therefore, the presence of a gas tube residue can become an obstacle, depending on the applications. In particular, in the case where the emission part is used as a light source of a projector, it is a major disadvantage that the residue occurs on an image screen as a shadow. Furthermore, in the quartz glass of the emission part, distortion of the glass occurs, since the gas tube is heat treated upon its removal. In the case in which this distortion is large, the emission part can break during luminous operation of the lamp.
On the other hand, to prevent the formation of a shadow on the image screen the lamp can be arranged with consideration of the positional relation to the screen. This process is, however, not especially desirable since it takes a long time for the lamp to be installed, and furthermore, since projector lamps are frequently replaced.
Based on the above described circumstances there is a need for a lamp in which there is no gas tube residue on the emission part.
On the other hand, a lamp is also already known in which the gas tube through which the gas is introduced and encapsulated is installed in one part of the tube arm. By means of this type of lamp, the above described disadvantage is eliminated, since there is no gas tube residue on the emission part.
In this lamp, it is necessary to provide a cavity which is connected to the inside of the emission part within the tube arm. In this case, therefore, it is not possible to obtain a hermetically sealed arrangement by melting the quartz glass at the base of the emission part.
In an arrangement in which the entire inner sides of the emission part and the tube and are formed as a cavity, and in which a lead is held only from one end of the tube arm, it is difficult to arrange the electrodes in the desired position as the result of the weight of the lead and the electrodes. Furthermore, holding of the lead is destroyed by vibrations and impacts in the transport of the lamp.
Therefore, a process is also known in which a part used in the tube arm especially for holding the lead in order to place the electrodes in the desired position. However, in this process the size of the gas passage is limited, and the arrangement of this holding part in the lamp with a hermetically sealed arrangement is not easily obtained.
Use of a perforated glass sphere is described as a process for holding the lead pin in German patent P 3029824.9. This perforated glass sphere is located within the tube arm in the vicinity of the emission part and is provided in part with an opening through which the gas can be discharged. This arrangement, however, has the following disadvantages:
The part which holds the lead consists of glass. However, processing thereof is not easily performed since the size thereof must be roughly the same as the inside diameter of the pipe arm, and since a gas passage opening is necessary. PA1 The perforated sphere consists of the same glass as the emission part. In lamp production, therefore, cracks easily occur in the emission part or in the perforated sphere. PA1 Furthermore, a pressing device is necessary to press the perforated sphere onto the side of the emission part. At the same time, there is the disadvantage that this pressing device is also heated as the temperature of the lamp rises, and under certain circumstances, loses its function due to deformation: PA1 If between the cross sectional middle of the robe arm and the cross sectional middle of the perforated sphere there is a fault, the position of the electrodes deviates from the desired position. When used over a long time, polarization of the electrodes occurs which can also be designated as concentration or accumulation on a certain side and wearing of the electrodes takes place; this causes unstable are discharge.